Advanced Level Quantitative Chemistry: Volumetric titration calculations 2 (non-redox)

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Doc Brown's GCE AS A2 A Level Chemistry - Advanced Level Chemistry Revision on Volumetric Titrations

GCE A Level AS-A2 IB Acid-base and other non-redox volumetric titration quantitative calculation questions

PART 2 Questions 21 onwards

In PART 2 of these A Level quantitative chemistry calculation questions  you have to work out the method a bit more on your own! I have not broken the problems down into 'mini-steps' for you, but request you use your experience from solving the PART 1 Questions. I have also included some 'gas volume' and 'gravimetric analysis' questions to provide more variation of quantitative analysis questions and standardising hydrochloric acid calculations.

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If you find these useful or spot a silly error please EMAIL query ? comment

I DO MY BEST TO CHECK MY CALCULATIONS, as you yourself should do,  BUT I AM HUMAN! AND IF YOU THINK THERE IS A 'TYPO' or CALCULATION ERROR PLEASE EMAIL ME ASAP TO SORT IT OUT!

Relative atomic masses that may be needed: H = 1, C = 12, O = 16, Na = 23, S = 32, Cl = 35.5, Ca = 40, Cu = 63.5, for questions 21 onwards

I've tried to quote the data to the appropriate significant figures and associated 'trailing zeros'.

Q21 Analysis of limewater.  25.0 cm3 aliquots of a calcium hydroxide solution were titrated with 0.100 mol dm-3 (0.100M) hydrochloric acid using phenolphthalein indicator. On average it took 10.50 cm3 of the acid to neutralise the limewater.

Calculate ...

(a) the molarity of the calcium hydroxide solution

(b) the concentration of calcium hydroxide in (i) g dm-3 and (ii) g/100 cm3

Q22 If it took 20.55 cm3 of 0.100 M HCl to neutralise 25.0 cm3 of an NaOH solution, calculate the molarity of the alkali.

Q23 A standardised solution of sodium hydroxide had a concentration of 0.1025 mol dm-3 (0.1025M). If 25.0 cm3 of a sulfuric acid solution required 17.65 cm3 of the NaOH to neutralise it, calculate the molarity of the acid.

Q24 Citric acid has the formula and is tribasic acid, forming the tri-sodium salt on complete neutralisation with sodium hydroxide.

A 25.0 cm3 sample of a concentrated citrus fruit cordial component (e.g. for lime juice), used in the food & drinks industry, was diluted to 250 cm3 in a graduated volumetric flask.

A 25.0 cm3 sample of this diluted solution, required, on average, 22.5 cm3 of a standard 0.100 molar sodium hydroxide solution using phenolphthalein indicator for the titration end-point.

Assuming all the acid in the cordial was citric acid, calculate the concentration of the acid in g/dm3 in the original solution.

Q25 4.28 g of a hydrated form of the salt sodium sulphate, Na2SO4.xH2O, was heated at 180oC until the remaining mass became constant at 1.89 g. Calculate x, the number of molecules of 'water of crystallisation'.

Q26 On reaction with dilute hydrochloric acid, 0.428 g of a group 2 metal (M) formed 75.0 cm3 of hydrogen gas at 298K/1 atm pressure.

If the molar volume of gas is 24.0 dm3 at 298K/1 atm, deduce the atomic mass of the metal and identify it from the periodic table.

Q27 To standardise a solution of hydrochloric acid, it was titrated against accurate masses of anhydrous sodium carbonate (which should be dried in an oven prior to use). e.g. 0.132 g of Na2CO3 required 24.8 cm3 of the hydrochloric acid for complete neutralisation, calculate the molarity of the hydrochloric acid.

Q28 Follows on from Q27. You can make up a solution of sodium carbonate from which you pipette 25.0 cm3 aliquots to obtain an accurate average titration value as an alternative to multiple weighings. 1.30g of anhydrous sodium carbonate was dissolved in 250 cm3 of de-ionised water in a volumetric flask. 25.0 cm3 aliquots were pipetted and titrated with a hydrochloric acid solution (in the burette) to be standardised.

(a) Calculate the molarity of the sodium carbonate solution.

(b) If the average titration was 24.35 cm3 of hydrochloric acid, calculate the molarity of the acid assuming the sodium carbonate is fully neutralised to sodium chloride.

Q29 The purity of anhydrous sodium carbonate can be determined by titration with standard hydrochloric solution. The following results were obtained on titrating accurately weighed amounts of sodium carbonate with 1.00 mol dm-3 hydrochloric acid (1.00 M HCl) to complete neutralisation given by the equation below.

Na2CO3 + 2HCl ==> 2NaCl + H2O + CO2

 Analysis Result Mass of Na2CO3 titration (i) 1.113 g 20.95 cm3 (ii) 1.092 g 20.55 cm3 (iii) 1.166 g 22.00 cm3

The titration values were recorded to the nearest 0.05 cm3 for a burette calibrated in 0.1 cm3 increments.

In each case calculate (a) the moles of HCl used in each titration, (b) the moles of Na2CO3 titrated based on the moles of HCl used, (c) the mass of Na2CO3 titrated based on (b), and finally, (d) from your (b) and (c) calculations, calculate the % purity of the anhydrous sodium carbonate, quoting what you think is the most accurate analytical result.

Q30 How to determine the water of crystallisation in hydrated sodium carbonate crystals ('washing soda') by titration with standard hydrochloric acid solution.

If 0.352g of hydrated sodium carbonate crystals, Na2CO3.xH2O, was titrated with 0.100 mol dm-3 (0.100M) standard hydrochloric solution. If the titration value was 24.65 cm3,

calculate the moles of Na2CO3 titrated and hence deduce x, the number of molecules of water of crystallisation in washing soda crystals.

Q31 Blue hydrated copper(II) sulfate crystals have the formula CuSO4.xH2O, where x is the number of molecules of 'water of crystallization'.

If 3.33g of copper sulphate crystals where heated at 150oC until the residue of anhydrous copper sulphate remained constant at 2.12g.

(a) Calculate the % water of crystallisation in the blue copper sulphate.

(b) From your answer to (a) calculate the value of x in the formula of the blue crystals.

Q32 It is possible to analyse a mixture of sodium hydrogencarbonate and anhydrous sodium carbonate, either as a solid or solution, by titration with a standard acid solution e.g. 0.1 to 1.0 mol/dm3 hydrochloric acid.

7.357 g of an solid anhydrous sodium carbonate (Na2CO3)/sodium hydrogencarbonate (NaHCO3) mixture was dissolved in deionised water and made up to 250 cm3 in a calibrated volumetric flask.

25.00 cm3 aliquots were accurately pipetted into a conical flask and titrated with 0.5000 mol/dm3 hydrochloric acid.

Using (1) phenolphthalein indicator (pKind = 9.3), the average titration was 9.80 cm3 of the HCl.

With (2) methyl orange indicator (pKind = 3.7), the average titration was 24.75 cm3 HCl.

(a)(i) On adding the acid to the alkaline carbonate mixture solution, what are the colour changes for the end-points of titration (1) and (2)?

(a)(ii) Give the equations for what happens in each titration.

(b)(i) Calculate the moles of Na2CO3 in the prepared solution and calculate its molarity.

(b)(ii) Calculate the mass of sodium carbonate in the solution and hence the percentage Na2CO3 in the original solid mixture.

Extra calculations for further practice and will partly help you to solve Q33

(c)(i) In the titrations, what total volume of the HCl was used to neutralise the Na2CO3?

(c)(ii) In titration (2) what volume of HCl was used to neutralise the NaHCO3 in the original mixture?

(c)(iii) Calculate the number of moles of NaHCO3 in the 25.0 cm3 aliquot at the start, and the total moles in the prepared solution.

(c)(iv) Calculate the mass off NaHCO3 in the prepared solution, and hence its % in the original solid mixture.

Q33 Sodium hydroxide solution can be used to absorb carbon dioxide gas.

2 dm3 (2 litres) of a sodium hydroxide solution was used to cleanse a gas mixture of carbon dioxide. The resulting solution X now consists of a mixture of unreacted sodium hydroxide and sodium carbonate.

Solution X was titrated with standard 1.000 mol/dm3 hydrochloric acid ...

(1) using phenolphthalein indicator.

(2) using methyl orange indicator and

From these titration values you can calculate the molarity of unreacted sodium hydroxide, the molarity of the sodium carbonate formed and how much carbon dioxide gas was absorbed.

Using 25.00 cm3 pipetted aliquots of solution X, on average ...

titration (1) with phenolphthalein gave a value of 12.45 cm3 of 1.000 mol/dm3 HCl

titration (2) with methyl orange gave a total value of 22.55 cm3 of 1.000 mol/dm3 HCl

(a) After some research, briefly explain why titration ...

(1) using phenolphthalein indicator corresponds to the end-point of the titration-half-neutralisation of NaOH and Na2CO3 to NaHCO3

(2) using methyl orange indicator corresponds to the end-point of the titration of the complete neutralisation of NaOH plus all of Na2CO3

How you can do the analysis with a single titration?

(b) Calculate the molarity of the sodium carbonate formed.

(c) Calculate the molarity of the unreacted sodium hydroxide.

(d) Calculate the volume of carbon dioxide gas absorbed (assume molar gas volume = 24 dm3 at RTP).

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